Port timing and size (area)

Mr_Cag · #1 12-16-2005

I’m porting my barrel to a better spec and know what I want to get it too for decent exhaust, transfer and blow down timing.

But what Port area?
As far as I can work out there is no hard and fast rule that the exhaust area must be ‘X’ amount bigger/smaller than the transfer area. All the engine specs I have looked at have ranged so much that I am not sure. I first thought the exhaust should be about a quarter/fifth bigger than the transfers, but then other engines have more transfer area then exhaust.
Can anyone suggest good area to start with?

Currently with the calculations I have done so far it stands at: Exhaust area = 296mm2, Transfer total area = 256mm2.

If its needed - it’s a 40mm bore x 32mm stroke.
Exhaust open at 94 Degrees ATDC - duration 172 Degrees
Boost port opens 119 degrees ATDC- duration 122 Degrees. 65 Deg roof angle.
Transfer opens 121 degrees ATDC- duration 118 Degrees. 0 Deg roof angle.
Blowdown 25 Degrees.

JDQuikSIR · #2 12-16-2005

why don;t you start with a smaller port job on the exhaust...and if it helps out a lot....widen it some more...and do your timings again..

do you have any plans with your transfer ports?

Mr_Cag · #3 12-16-2005

Thats kind of what i'm doing. I'm putting in the exhaust the size i've stated but have room to take it bigger, prob another 40mm2 if needed. Obviously i can't make it smaller once its done tho, so was wondering if anyone knows approx the diffencence in area between the transfers (including boost) and exhaust before I start removing metal. There just doesn't seem to be any clues in anything i've read. Anyone have access to a tuning program that can help calculate it?

Quote:

do you have any plans with your transfer ports?

The transfers have to be taken higher to get the correct timing. A boost port is being added too.

d.king · #4 12-16-2005

I am new to this and am trying to learn every thing i can. Can you tell me what blowdown is??

I have a good running motor now and was gonna get a deg wheel and check to see where the timming is now. This i can use as a base.

Thanks
DK

JDQuikSIR · #5 12-16-2005

okay....do you have a head kit? Or just normal 40mm head...? If you got a single piece cylinder head...I would just buy a couple more just to try different formulas in mm2 and see what works best for you. Theres no tuning program as far as I know...but usually, everything about upping the umph in our cags is trial and error....

d.king · #6 12-16-2005

Just a stock head for now.

Soo, no hard numbers then??

DK

bartw · #7 12-16-2005

http://edj.net/2stroke/jennings/2str...rshandbook.pdf

This will answer many questions. I don't have the software to actually see what will work best for you. I can tell you that my current setup includes a "headkit" which allowed me to make a small aluminum base plate spacer and still retain the compression while effectively raising the transfers. Go wide on the exhaust. I did not raise mine since the base plate already raises everything but did lower it a little. It has been too long since I did the math on what the actual timing on that motor is but I might just throw the wheel back on it and let you know. (BTW, I am also running a full circle stroker crank so my timing is already different than a stock set up)

d.king · #8 12-17-2005

bartw......Thank you!!! I was looking for that!!

DK

Mr_Cag · #9 12-17-2005

Thanks all for your replys. I've read a lot of the tuning stuff but the port areas although, always mentioned, it would seem that there is no simple rule/guide to working it out.
I just thought maybe someone had run the CAG engine (with a similar spec to mine) through a program to work it out?

Mr_Cag · #10 12-17-2005

I don't have a head kit (it's all standard-40mm) so can't just look into the barrel and at a degree wheel and see where the ports open, but worked out all the spec using CAD. I know the piston position relative to the bottom of the exhaust port, i know the stroke (32mm) , sqiush (0.9mm) and con-rod lenght so can work out the rest from there.
I'm not at work now but if i remember right - my engine, as it came out the box, had 130mm2 transfer area and 190mm2 of exhaust.
Exhaust opened at 108 degrees ATDC
Tranfers opened at 130 degrees ATDC
So blow down (time for exhaust gasses to escape before the tranfers open) was only 22 Degrees!

A good general guide for exhaust, transfer and blow down timing is (plus or minus 5 degrees):
Exhaust= 90 degrees ATDC
transfers= 120 degerees ATDC
Blow down 25-30 degrees ADTC

So my engine standard the exhaust opens nearly 20 degrees to late! has too low a blow down time, and the transfers aren't open long enough buy approx 20 degrees (duration).
It really is quite a poor spec!

On a engine such as the cags where the stroke is only 32mm 2 degrees on the crank equates only to about 0.4mm of stroke! so if you're messing with your timing - you have to do it accurately! E.g. If you raise the roof of any port by only 0.5mm you are changing the timing by approx 3-6 degrees, which is a lot!

I'm fortuate that I am a Toolmaker for a large company so have access to cutting edge machines

I can machine in the ports to the nearest micron (1000th of a millimeter!) which is nice. I'll post some pics of the process as i do it for anyone who's interested.

bartw · #11 12-17-2005

My crank is 35mm, it is the full circle crank that also takes up a lot of the useless case volume. My aluminum base plate spacer was the only cheap way I knew to raise my transfers and increase their duration, I just don't have access to the kind of machinery you are talking about and if I had someone do it for me it would not be economical.

cagaroni · #12 12-17-2005

definitely post some pics ....

d.king · #13 12-17-2005

I second the pics. Would love to see them.

also, the timming listed is what i was looking for. Genaral guide lines. Now i can get my deg wheel and check my motor.

I am a machinist by trade. It's just bridge ports and laths but, i can still get in to .001 of an inch. Maybe after i find out where mine is at i can figure out how to fix it if its not. Or just make it better! hehe.

DK

d.king · #14 12-17-2005

ok, just checked my motor.

Exhaust=102 deg
Transfer=122 deg
Blow down=20 deg
Squish= 1.1mm

I bet if i raise the exhaust roof about .5mm it should improve a bit. I am stuck with the squish for now. Would like to get it down to .7 or .8mm but, dont know if i could raise the ports correctly. Have to check at work and see what cutters i can get my hands on.

DK

JDQuikSIR · #15 12-17-2005

those seem like some high squish numbers.... to make some decent power and wake these engines up... you need to be around the .47mm - .55mm range and for sure you would have an increase in power and a more snap in powerband. I made my own base gaskets to get my .47mm squish and that made a huge difference...

#16 12-17-2005

Im using a 2mm base spacer like bartw said, and I have

Ex Duration: 170
In Duration: 140
Squish: .45mm

It runs really strong mid to top (40mm piston), the low is not great but thats what the adjustable clutch is for.

d.king · #17 12-17-2005

If i drop the gasket, the squish will get better(.5 to .6mm) but, wont the timming being off cancel any effect the squish had?!?!

DK

caferacer · #18 12-17-2005

If I had to choose between altering the port timing and the squish I would take the lower squish any day.

JDQuikSIR · #19 12-17-2005

Quote:

Originally Posted by caferacer

If I had to choose between altering the port timing and the squish I would take the lower squish any day.

hell yea! Squish is the way to go.....

d.king · #20 12-17-2005

Ok, i have 1.1 to 1.2mm squish. The base gasket is .5mm. That will leave me with a squish of .6 to .7mm. From what i read, that will put my power band in the mid range. Right?

DK